|Alternative Title||Mode-Competition and Predictions of the Instability of Deepwater Submarine Pipelines|
|Place of Conferral||北京|
A long-distance deepsea pipeline usually crosses different water areas, and encounters various environmental loads and seabed geological features. As such，a reasonable assessment of the pipeline stability and potential risks is of great significance for the laying and safe operation guarantee. This dissertation mainly focuses on the lateral-instability of pipe, the tunnel-erosion of seabed soil, and the lateral global buckling of submarine pipelines. Its aim is to obtain fluid (thermal)-structrure-soil coupling mechanisms of pipeline instability, to reveal the mode-competition mechanism between the lateral-instability of pipe and the tunnel-erosion of soil, and eventually establish a semi-analytical analysis method for predicting the lateral global buckling of long-distance pipelines, which can reflect random characteristics of the spatial distribution of seabed properties.
By analyzing the microstructure, the physical and mechanical properties of columnar surface sediments at certain deepsea locations of the South China Sea, we found that the microstructure of such deepsea sediments is flocculated or laminated, and that characteristic features of the sediments are high microbial content, high void ratio and low strength, etc. Based on the passive soil pressure theory, an analytical model is then developed for predicting the pipeline lateral instability on the clayey seabed. In the proposed model, the ultimate lateral soil resistance can be decomposed into a passive-pressure component and a sliding-resistance component. Through the flow-pipe-soil sequential coupling analysis, the competition mechanism between the lateral-instability of pipeline and the tunnel-erosion of the underlying soil is revealed. The instability envelope for the pipe-soil interaction system is further established, which can be described with three key parameters, i.e. the embedment-to-diameter ratio, the dimensionless submerged weight of pipe, and the corresponding critical flow velocity. Parametric study indicates that the tunnel-erosion is more prone to emerge than the lateral-instability for small values of embedment-to-diameter ratio. With increasing pipe embedment, the tunnel-erosion could be suppressed, and the lateral-instability thereby gets more preferential for occurrence.
Since the stability of deepsea pipelines is always affected by the sloping seabed at the continental slopes, the axial pipe-soil interactions along a sloping sandy seabed are investigated. By analyzing the results of the existing mechanical-actuator experiments, the basic characteristics and trends for the relationships between the axial anti-sliding capacity and its corresponding axial pipe displacement are obtained. Considering the fact that the integrated normal pipe-soil contact force would exceed the submerged weight of the pipe due to the effect of pipe curvature, a wedging factor for the sandy seabed is derived. An expression for the ultimate axial anti-sliding capacity is finally deduced.
According to the exploratory and experimental data of deep-sea surface sediments from the South China Sea, we carried out the statistical analysis of the spatial random distribution characteristics of sediments properties. Based on the random field theory, the construction of spatial random field model describing the soil properties is realized by employing the Local Average Subdivision algorithm. Then, a semi-analytical stochastic finite element model for predicting the lateral buckling of submarine pipelines is proposed, which incorporates the spatial random distribution characteristics of soil properties. With the proposed model, simulations and reliability analysis are performed for the lateral global stability of a submarine pipeline on heterogeneous seabed. The Monte Carlo simulation results demonstrate that, the mean value of critical temperature for a heterogeneous seabed is slightly smaller than that for a homogeneous seabed. Furtheremore, the critical temperature and the variation coefficient of undrained strength approximately satisfy a linear relationship. Compared with the present reliability analysis, the traditional deterministic analysis could predict the lateral buckling of the pipeline with a relatively high probability.
|师玉敏. 深水海底管道典型失稳模式竞争机制及失稳预测[D]. 北京. 中国科学院大学,2019.|
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